Calcium phosphate (Ca—P) materials commercially available as bone graft or bone substitute materials for dental and orthopedic applications include calcium phosphate phases such as: hydroxyapatite (HA), Ca10(PO4)6(OH)2; beta-tricalcium phosphate (β-TCP), Ca3(PO4)2; biphasic calcium phosphate (BCP) consisting of mixtures of HA and β-TCP; unsintered apatite or calcium-deficient apatite (CDA); coralline HA, and bovine bone derived apatite (sintered and unsintered). These materials are characterized as bioactive, osteoconductive, and promote direct attachment with bone without intervening fibrous tissues, thus developing a very strong interface between the Ca—P material and bone. However, a serious shortcoming of Ca—P materials is their low mechanical or fracture strength and they therefore cannot be used in load-bearing areas.
Commercially pure titanium (cp-Ti) and titanium (Ti) alloy (Ti6Al4V) are metals possessing corrosion resistance, biocompatibility, durability, and strength. These metals are preferred for dental and orthopedic implants or prosthesis. However, these metals do not directly bond to the bone. ‘HA-coated’ dental and orthopedic implants were therefore developed to combine the bioactivity and osteoconductivity of the ‘HA’ coating and the properties (e.g., strength) of the Ti or Ti alloy substrate. Plasma spraying is the coating deposition technique used for the commercial ‘HA-coated’ orthopedic and dental implants. This technique uses HA as the coating source and involves extremely high temperature. The high temperature and other operating parameters produce coatings of variable composition, principally in the ratio of the crystalline (principally HA) to non-crystalline (amorphous calcium phosphate, ACP) phases (HA/ACP ratio). This ratio was found to vary from 30HA/70ACP to 70HA/30ACP in coatings of commercial implants. Of the crystalline phase, 90 to 95% is HA and 5 to 10% is made up of mixtures of tricalcium phosphates (α-TCP, β-TCP), tetracalcium phosphate (TTCP), and sometimes, calcium oxide, CaO. The coating composition (mainly the HA/ACP ratio) significantly affects in vitro dissolution properties of the coating: the lower the ratio, the more soluble the coating. The variability in coating degradation may affect biological performance, coating stability and implant stability. It is therefore necessary to develop alternative coating methods using low temperature that will provide coatings with reproducible homogeneous composition.
Deposition of brushite or dicalcium phosphate dihydrate (DCPD), CaHPO4.2H2O, dicalcium phosphate anhydrous (DCPA) or monetite, CaHPO4, and apatite (AP) has been achieved using electrochemical deposition method AP coatings have also been obtained by transformation of the initially formed DCPD or DCPA coating. In studies relating to these, neither the adherence of the ECD-deposited calcium phosphate coating to the substrate nor the dissolution properties of the coatings was reported.
Octacalciumphosphate (OCP), Ca8H2(PO4)6.5H2O, one of the biologically relevant calcium phosphates, can easily transform to carbonate hydroxyapatite (CHA) in synthetic systems. Because of structural similarity between OCP and HA, Ca10(PO4) 6(OH)2, OCP is speculated to be a necessary precursor of bone apatite which is a carbonate hydroxyapatite (CHA). OCP has been demonstrated to be more resorbable and to enhance more bone formation than either HA or β-TCP.